Unresolved dependency warning includes source position. Fixes #528

Unresolved dependency warning is moved to UnresolvedDependencyWarning class including
the fail path that was added in #1467.
To display the source position, I need to access the State, so I had to move the
error processing out of IvyActions and add UnresolvedDependencyWarning, which is
aware of State.
Adds UnresolvedWarningConfiguration that caches ModuleID -> SourcePosition mapping.

UnresolvedWarning is moved back to IvyActions.scala where it belongs.
The mapping between ModuleID and SourcePosition is passed in as UnresolvedWarningConfiguration.
This is calculated once in Defaults using State and is cached to filesystem.
Adds `dependencyPositions` task to explicitly track dependency positions.
Disable flaky test, as it's timeout based.

Fixes #1398
Use HTTPS for downloading artifacts from Maven Central

Sonatype have enabled HTTPS access for Maven Central:

http://central.sonatype.org/articles/2014/Aug/03/https-support-launching-now/

Note that the Ivy class IBiblioResolver contains the old http url
(ie DEFAULT_M2_ROOT="http://repo1.maven.org/maven2/"):

http://svn.apache.org/viewvc/ant/ivy/core/trunk/src/java/org/apache/ivy/plugins/resolver/IBiblioResolver.java?revision=1557968&view=markup#l72
Update misc tests and doc to reflect HTTPS access for Maven Central
scalariform project/Docs.scala
Adds sbt.repository.secure (default: true) #1494
Fix pom-export of Maven Central repository

Commit a1e26ca6 broke the `make-pom` & `pom-advanced` dependency-management
tests by replacing one reference to `IBiblioResolver.DEFAULT_M2_ROOT` in
`Resolver`, but not the other reference in `MakePom`:

https://travis-ci.org/sbt/sbt/jobs/31939788#L2517-L2519

...the secure url was no longer recognised as the default Maven Repository
root, so was erroneously exported.
Merge pull request #1507 from sbt/wip/optional-https-maven-central

"Use HTTPS for downloading artifacts from Maven Central" + disable flag
Merge pull request #1515 from sbt/wip/fix-1398

Disable flaky test, as it's timeout based.
Merge pull request #1491 from sbt/fix/528

Unresolved dependency warning includes source position. Fixes #528
Allow root plugins to be disabled.

Fixes #1455

* Add a mechanism to detect if a plugin clause includes/excludes
  a particular plugin.
* Add a filter for our "enabling" clauses so that user-excluded
  root plugins do not show up.
* Add a test to ensure this succeed.
Added notes so we have a merge conflict for @eed3si9n to deal with.
So apparently scalariform was unable to parse these files.